In the inventions relating to a conventional idle-stop system, restart is performed by bringing a pinion gear of a starter and a ring gear into meshing engagement with each other during rotation of an engine in order to improve restartability after an engine stop operation is performed. For example, the rotation of a starter motor is started in response to a restart request for the engine. A rotation speed of the starter motor is brought into synchronization with that of the engine by energization with speed governing, thereby coupling the starter motor and the engine to each other (see Patent Literature 1, for example).
Moreover, when the engine is stopped, even in the case where the restart request is not issued, the pinion and the ring gear are being brought into meshing engagement while the engine is rotating or immediately before the engine is stopped, for preparation of the restart request which may be issued at any time (see Patent Literature 2, for example).